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I expect you have read one or to papers on cryptography...like everyone else
You read, looked confused, felt confused, and a few probably ran away and Cried.
Why? Because the encryption used today"s is industrial strength. That"s Because
its has to be as our lives rely on information, and so do businesses, So encryption
has to stronger than an iron sponge soaked in super glue to keep Our secrets
safe.
So any papers on the subject start at the top because that is the standard
today. But I will start at the beginning (that"s original) so you can learn
the Basics of cryptography and build from there.
The overview
Here is a simple diagram of how the encryption system works...
Plain text - what you wrote
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Encryption
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Cipher text - what it looks like after encryption...so no one can read it
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Decryption
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Plain text - again
So there are five stages of encryption...some of you will now be shouting at
Your screens that I am wrong...as there are keys and stuff...I will talk about
This in another paper as this is advance stuff :o,
Monoalphabetic substitution
The very basic of cryptography is that horrible long word written above you.
What I mean is that you replace the letters of the alphabet with other Letters.
For example...
a b c d e f g h i j k l m n o p q r s t u v w x y z = plain text
c d e f g h i j k l m n o p q r s t u v w x y z a b = cipher text
So if i wanted to write the message "script kiddies like tuna" i
would write...
Script kiddies like tuna
uetkrv mkffkgu nkmg vwpc
Looks like good encryption doesn"t it? Could you crack it? Don"t worry...i
will Tell you how later :o) This method of simply shifting the alphabet is called
the Caesar Cipher...guess who it was invented by :o? To crack this encryption
is easy...because once you have found one letter you Can just unshift the cipher
text. For example this cipher is just shifted two Spaces to the right. So this
encryption is weak. We can make it slightly Stronger by assigning each letter
another random letter...like this.
a b c d e f g h i j k l m n o p q r s t u v w x y z = plain text
w q r e t y u i o p a s d f g h j k l z x c v b n m = cipher text
This is slightly stronger as when you get one letter...it doesn"t unlock the
Rest. So how do you crack it?
Cracking a monoalphabetic substitution cipher
That is a very long scary word...even to me :-|
So how do you crack this *cough* super strong cipher? We use a thing called
Frequency analysis...because you see in the English language each letter has
a personality...and appears a certain number of times. For example the most
Common letters in the English language are E, T and A. so if i took a piece
have Cipher text...and counted all the letters and took a percentage...and compared
To the average percentage...i can decrypt the letter.
Here is the table of frequency analysis for the English language...
a - 8.2%
b - 1.5%
c - 2.8%
d - 4.3%
e - 12.7%
f - 2.2%
g - 2.0%
h - 6.1%
i - 7.0%
j - 0.2%
k - 0.8%
l - 4.0%
m - 2.4%
n - 6.7%
o - 7.5%
p - 1.9%
q - 0.1%
r - 6.0%
s - 6.3%
t - 9.1%
u - 2.8%
v - 1.0%
w - 2.4%
x - 0.2%
y - 2.0%
z - 0.1%
So if i had a piece of encrypted text...i could count all the letters...then
i would assume the most common letter in the cipher text...say p...is really
e...and the next most common say l... could be t. You do this all the way thought
and you should start to see words forming. There is alot of guess work and fill
in the blanks. Also trail and error as the most common letter in the text may
not be e...but a...so you have to think on you feet.
The only problem with frequency analysis is that the encrypted text has to
at least be over a hundred words...because if it is short the averages are all
screwed...and using frequency analysis would be hard or impossible as some letters
would not match there percentages.
Another way to crack it would be to look for cribs. For example we have the
following encrypted word-
lppg
What could it be?? Frequency analysis wont work as it is too short. But look...a
double letter. We can use this as a crib as there are only some letters which
can be doubled, these are...
ss
ee
tt
ff
ll
mm
oo
So the middle two letters must one of these pairs...we can narrow it down as
there are few or no words in the English language which and four letters long
and have to constanents in the middle... so it must be either-
oo
ee
So we have narrowed down a letter to one of two possibilities...together with
the rest of the cipher text there would be more clues to these letters identity.
Another cribe is single letters...take this for example-
plmk r qsrf
This cipher text has a single letter in...and this letter must be either A
or I.
Other cribs
The most common three letter words in the English language are-
and
the
the most common two letter words are-
of
to
in
it
is
be
as
at
so
we
he
by
or
on
do
if
me
my
up
an
go
no
us
am
Conclusion
Well there you go...you should hopefully understand the very basic of encyption...
I will explain other forms of encryption in other papers...
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